after soaring radiation forces a retreat. and the u.s. tells americans to evacuate a 50-mile danger zone. i'm katie couric. also tonight, the question everyone in this country is asking: could it happen here? the u.s. has 23 nuclear reactors just like those in japan. how safe are they and we? and as the search goes on for victims of the earthquake and tsunami, an american exchange teacher is among the missing. captioning sponsored by cbs from cbs news world headquarters in new york, this is the "cbs evening news" with katie couric. >> couric: good evening, everyone. they have what could be the most dangerous job in the world, and the world is rooting for them to get it done. the nuclear power plant workers in japan trying to prevent a meltdown. radiation at the dai-ichi plant in fukushima got so high today they were forced to leave temporarily, but now they're back on the job. japan has raised the maximum radiation dose allowed for nuclear workers so they can deal with the crisis, but the head of the u.s. nuclear regulatory commission told congress today the doses those workers could be

with the crisis, but the head of the u.s. nuclear regulatory commission told congress today the doses those workers could be exposed to are potentially lethal in a short period of time. it's nearly six days now since the earthquake and tsunami killed at least 4300 people and damaged the nuclear reactors. today, u.s. officials told americans within 50 miles of the plant to evacuate the area or stay indoors. that is two and a half times as wide as the danger zone established by the japanese. harry smith begins tonight's coverage of the disaster in japan. >> reporter: in a sign of how grave japan's crisis has become, the emperor, akihito, made an unprecedented television address, acknowledging that he is deeply worried, urging his subjects not to give up. it did little to calm a country increasingly distrustful, given the wave of conflicting reports and mixed messages. >> ( translated ): there is both positive and negative news. i don't know which i should believe. >> reporter: and today on capitol hill, u.s. energy secretary and nuclear expert steven chu said he, too, is baffled. >> and ther

, protests, and more deadly violence in libya as qaddafi's forces battle the rebels in several key cities, u.s. aid arrives in tunisia for refugees fleeing the violence. the tragic death of a star high school athlete just moments after he leads his team to victory. and they call this the most dangerous eight seconds in sports. captioning sponsored by cbs from cbs news world headquarters in new york, this is the "cbs evening news" with katie couric. >> couric: good evening, everyone. president obama is keeping his exuberance in check over today's news about unemployment. he calls it progress. the unemployment rate-- 9% or higher for a record 21 months-- has finally dropped below that mark, falling last month to 8.9%. and the pace of hiring is picking up. the economy added 192,000 jobs. anthony mason is our senior business correspondent and, anthony, the recession has been officially over for months now. finally it looks like the job market is catching up. >> reporter: after months of disappointingly weak numbers, katie, the labor market is finally flexing some muscle. in wisconsin this week, th

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